These days electronic products have become increasingly personalized and multifunctional. Activities such as listening music, watching movies and chatting via videos through electronic products are increasingly popular among many people. In order to get full and improved sound quality and avoid affecting other people most people would do the aforesaid activities by using headphones. However, the sound quality in the headphones could suffer due to too much ambient noise. To resolve this problem noise reduction headphones that can reduce ambient noise have been developed and increasingly accepted by consumers. At present the noise reduction techniques adopted in the electronic products mainly can be divided into active noise reduction and passive noise reduction.
The headphone of the conventional passive noise reduction technique aims to reduce external background noise. For instance, Taiwan patent No. M359891 entitled “Noise-proof headphone” discloses a noise-proof headphone which includes a hanging arm and a shell. The shell is located at two ends of the hanging arm and includes a shell cap and a shell body that form a housing space between them to hold a circuit board and a loudspeaker. The circuit board is formed in a shape mating the shell cap to couple tightly therewith so that the housing space is divided into a plurality of air chambers to isolate external noise.
Since the aforesaid noise-proof headphone has the circuit board and the shell cap tightly coupled together, a sound isolation wall is formed to isolate low frequency audio signals so that low frequency sound passing through the shell can be blocked by the sound isolation wall and the air chambers to reduce the external low frequency noise. However, in the event that the background sound is excessively large the sound in the headphone can still be affected, hence the aforesaid method of isolating the external background sound by blocking is limited in effectiveness.
Because of the efficacy deficiency of the passive noise reduction technique, most products in the market at present adopt the active noise reduction technique to reduce the external background sound in the headphone. For instance, U.S. Pat. No. 8,077,874 entitled “Active noise reduction microphone placing” discloses an active noise reduction microphone that has an error signal gone through a feedback process to combine with an audio in a signal processor, then is fed to a compensator which provides a phase input to an amplifier, then is fed to a sound module to combine with a noise; and through a microphone and an output amplifier, the error signal is formed which is combined with the audio through the feedback process. The above process is repeated cyclically to reduce the noise.
Through the sound module that generates the phase the noise can be offset and reduced, and through the feedback process the noise not being fully eliminated enters a next cycle of the noise reduction process. However, the noise is directly input to combine with the audio without going through any prior process but is reduced through a single loop, the noise reduction effect still leaves a lot to be desired. Moreover, it has merely one sound outlet that makes output sound monotonous and lower in quality. There is still room for improvement.